1. Field of the Invention
The present invention relates to a crosslinked thermoplastic elastomer blend composition superior in the balance of rigidity, impact resistance and processability, and in particular, resistance to elevated temperatures and the ability to maintain a significant percentage of its tensile strength, and elongation, upon exposure to heat. In particular, the present invention is directed to the preparation of a crosslinked thermoplastic elastomer blend formulation which is suitable for use in either an electrical insulating or tubing and hosing application, and which displays unexpectedly high resistance to heat aging and/or flame, good chemical resistance as well as possessing highly satisfactory strength characteristics over a broad temperature range.
2. Description of Prior Art
There remains a continuous demand, especially in the electronics industry, for thin-wall, truly low-cost wire or cable insulation exhibiting high electrical integrity and good physical properties. Towards this end, two specific resins, poly(tetrafluorethylene) and crosslinked polyethylene, have been popularized (along with numerous others, e.g. fluorinated ethylene-propylene resins and ethylene/tetrafluoroethylene copolymers) which when used alone, or combined (i.e. layered) exhibit many of the desired properties for an insulation application. See, e.g. U.S. Pat. No. 3,546,014.
That is, recognizing the virtues of both polyethylene and fluorocarbon type polymers, electrical insulation materials have been developed that combine both a polyolefin and a fluoropolymer to prepare a composite electrical insulating material with advantageous performance. For example, in U.S. Pat. No. 3,269,862, it was reported that polyolefins such as polyethylene are excellent insulating materials for electrical wires, electric components and the like. However, the excellent dielectric properties of polyolefins were said to be offset by the relatively low melting points and their low resistance to flame and oxidation. It was, therefore, pointed out that considerable efforts had been directed to developing polyolefin formulations which were not subject to such deficiencies. Accordingly, the relatively low melting point of polyethylene was reportedly improved by crosslinking, e.g., by irradiation or chemical means. In addition, it was further found that certain additives will flame retard polyethylene. However, many flame retardant additives adversely affect the dielectric properties of polyolefins as well as low temperature performance and corrosion resistance. The '862 disclosure went on to report, therefore, a composite electrical insulating material comprising a crosslinked polyolefin layer and a crosslinked polyvinylidene fluoride layer which in combination possessed a high degree of flame resistance and a high degree of resistance to heat aging and high strength characteristics over a broad temperature range. The composite electrical insulating material disclosed therein was found useful for insulating electrical wire and electrical components with excellent dielectric properties with respect to the coated wire substrate.
Building on the concept of a crosslinked polyolefin base layer, followed by a fluoropolymer outer coating, wherein the low melting point and low resistance to flame and oxidation of the polyethylene layer is compensated by crosslinking the polyethylene and coating with a fluoropolymer, a variety of disclosures have been made for producing materials suitable for insulation of wires and electronic components and reference is made to the following U.S. Pat. Nos.: 3,763,222, 3,840,619, 3,894,118, 3,911,192, 3,970,770, 3,985,716, 3,995,091, 4,031,167, 4,155,823 and 4,353,961.
In addition, and of more recent report, is U.S. Pat. No. 5,281,766 which describes lead wire for use in motors, coils and transformers, covered first with a layer of a primary insulation material including a crosslinked polyolefin followed by a second insulating jacket comprising the specific fluoropolymer: poly(vinylidene fluoride) or a poly(vinylidene fluoride) copolymer.
Accordingly, with the long-standing emphasis on a fluoropolymer type insulation, the long-standing question became whether or not such relatively expensive fluoropolymer systems could be replaced by a different thermoplastic resin composition which could be crosslinked to a desired level, without sacrificing properties, particularly the balance of tensile strength and elongation, subsequent to standard heat aging requirements for insulative materials. That being the case, a review of the prior art was conducted to ascertain what types of disclosures had been made with respect to crosslinked thermoplastic resin compositions that might be suitable for replacement of the previously described fluoropolymer materials.
For example, in U.S. Pat. No. 5,248,729, there is described a thermoplastic resin composition prepared by heat treating and crosslinking a mixture comprising (a) a thermoplastic resin containing no olefinic unsaturation and (b) an elastomer having an olefinic unsaturated carbon-carbon bond, for example, styrene-butadiene-styrene block copolymer. In particular, a dihydroaromatic compound was used as the crosslinking agent in crosslinking a mixture of a saturated thermoplastic resin and an unsaturated elastomer. The crosslinking was said to proceed only within the unsaturated component, with no substantial change of the saturated thermoplastic resin. The produced compositions were said to be superior in the balance of rigidity, impact resistance and moldability.
In U.S. Pat. No. 5,149,895, there is described a vulcanizable liquid composition which comprises a styrene-diene-styrene block copolymer, which is then crosslinked to provide a vulcanizable composition. The polymers produced were said to have high elongation and excellent aging characteristics.
U.S. Pat. No. 5,093,423 described a method for making styrene-butadiene thermoplastic elastomers. Specifically, the dynamic vulcanization of the styrene-butadiene elastomer is reported, along with a co-continuous matrix of styrene-ethylene-butadiene-styrene block copolymer and polypropylene. The dynamic vulcanization step is indicated to take place under appropriate conditions of sheer and temperature. Compositions of superior properties were reported to be achieved using this particular method.
U.S. Pat. No. 4,927,882 describes a styrene-butadiene thermoplastic elastomer which is said to be produced by dynamic vulcanization of the styrene-butadiene component.
U.S. Pat. No. 4,371,663 describes certain styrene polymer/thermoplastic elastomer blends made by melt-blending of styrene polymers and thermoplastic elastomers followed by a heat initiated crosslinking reaction along with the use of organic peroxides. Noteworthy improvements in ESCR, tensile strength, and practical toughness were said to be among significant physical properties improved in such polyblends.
Finally, it should be noted that previous work known to the present inventors related to blending together styrene-ethylene-butylene-styrene block copolymers and high and low density polyethylenes, in the presence of a crosslinking agent, followed by crosslinking to low levels of crosslink density. That is, these formulations contained a crosslinking density below that of the present invention and without the unexpected properties now claimed.
In sum, therefore, all of the above formulations were said to improve certain specific mechanical properties of the resulting materials, but none reported on the development of high levels crosslinking, or relative high thermoset character, while at the same time maintaining requisite flexibility for a wire coating, tubing or hosing application. That is, none of the prior art formulations described above report on material systems that can be made thermoset to a desired degree, without sacrificing the combined critical performance characteristics necessary for an insulating material: high tensile strength, sufficient flexibility and thermostability. In short, the prior art has not been totally successful in preparing a crosslinked thermoplastic elastomer suitable to replace some of the more expensive materials used in the insulated wire tubing or hosing industries.
Accordingly, it is an object of this invention to provide a crosslinked thermoplastic elastomer which is suitable for use as an insulating layer in the wire coating industry and which is useful in other applications where high tensile strength and flexibility are required, such as cable jackets, tubing and hosing.
More particularly, it is an object of the present invention to crosslink thermoplastic elastomer block copolymers, wherein the crosslinking is carried out to a level wherein flexibility is uniquely and surprisingly preserved, high tensile strength is maintained, and wherein both the tensile strength and flexibility remain in the crosslinked resin subsequent to long-term thermal aging.
Finally, it is a more specific object of the present invention to develop a high tensile strength yet flexible wire coating, tubing and hosing material stable to long term heat aging suitable for wire insulating applications, by the process of blending a thermoplastic elastomer block copolymer with a thermoplastic polymer resin, and crosslinking, wherein crosslinking is specifically developed as between the thermoplastic elastomer and thermoplastic polymer resin, at a desired level, optionally in the presence of a crosslinking promoter, along with incorporation of antioxidants and heat stabilizers.